Structural, Spectral, Biological and Electrochemical Studies Of Some ...

thiol equilibrium which in turn is influenced by the solvent and pH of the preparative
medium Many of the reported complexes have been prepared in mixed aqueous
solvents, often with bases added. However, there are few reports [22] in which
workers have varied the nature of their preparations to explore more about the
potential diversity ofthese ligands
The most common stoichiometries encountered with 2-heterocyclic
thiosemicarbazones are six coordinate having the general formula ML o + , where M is
2
Fe(III), Co(III) and Ni(II) L is tridentate, anionic ligand and n=O,.I, and planar with
the stoichiometry of MLX, where M=Ni(II) or Cu(II), L = tridentate, anionic ligand
and X is generally a halo or pseudo halo ligand. In addition to this there are reports
on the mixed ligands complexes of Mn(II), Cu(II), Ni(II) etc and even homo and
hetero bimetallic complexes [23]. Moreover it is also possible to isolate complexes
containing two non equivalent ligands-one protonated and the other deprotonated
with in the same coordination sphere. Recently there are reports on complexes with
tetra and penta dentate N2S2, N2S0, and N3S2 donor ligand in the monoanionic or
dianionic states having prounced anticancer activity [24].
HSAB considerations dictate that the oxidation state of a metal affects the
degree of its softness character and this is found to be stronger for transition metal in
the lower oxidation states [25]. Thus the low spin d 8 ion of Pd(II), Pt(II) and Au(III)
and d lo ions Cu(I), Ag(I) Au(I) and Hgill) exhibit higher stability constants with this
class of sulphur ligands because ofthe formation of strong sigma bonds as well as d1t
- d1t bonds by donation of a pair of electrons to ligand. It is reported that pyridine-2­
aldehyde Schiff base forms square planar complexes having the formula [M(NNS)X]
where M = Ni, Cu, Pt; and X =a simple or polyatomic anions, octahedral complexes
having the general formula [M(NNS)2]. Certain manganese complexes are reported
to have octahedral and polymeric structures [26]. Iron forms complexes having the
formula [Fe(NNS)2]CI04 [Fe(NNS)2][FeCI4]. Existence of two types of iron is
confirmed with Mossbauer spectral studies.
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